Traveling wave electron discharge devices



INVENTOR JOHN H. BRYANT ATTORNEY Feb. 4, 1958 J. H. BRYANT TRAVELINGWAVE ELECTRON DISCHARGE DEVICES Filed oct. 31, 1952 2 sheets-sheet 1 I lI I l l I I I I I l l I I I I I I I l I I I III l I I Il I I I I I I I II I l I l I I I I I l lllll n 2 f .fr llllllllllllllll IIIAIIIIIIIIIIIII IIIII l IIIIIIIIIIIIIIIII II I l l I l I I l I l l l |II\I I l I I I I l I I II|IIIIIIIIIIII|||I|III l/ IIIIIIIIIIIIIIIIIAIIIIIIIIIIIIIIII..." f /a/ f, I f fr. J n \\uf/nu/ W /`f/ 2 III! vFeb. 4, 1958 -J. H. BRYANT TRAVELING WAVE ELECTRON DISCHARGE DEVICESFiled oct. s1, 1952 2 Sheets-Sheet 2 MAGNET LENGTH MAGNET LENGTH T-*INVENTOR ,10H/v H. BRYANT BY /Q i;

ATTORNEY nited States Patent C TRAVELING WAVE ELECTRON DISCHARGE DEVICESJohn H. Bryant, Nutley, N. J., assignor to International Telephone andTelegraph Corporation, a corporation of Maryland Application ctober 31,1952, Serial No. 318,060 9 Claims. (Cl. S15-3.5)

This invention relates to traveling wave electron discharge devices andmore particularly to electron-optical systems of a permanent magnet typeapplicable to such devices.

The traveling wave type of tube is particularly useful in widebandmicrowave systems since it is capable of amplifying radio frequencyenergy over an unusual wideband of frequencies. The tube includes a formof transmission line, usually a helix, for transmission of microwaveenergy for interaction with an electron beam closely associated with theline. The helical characteristic of the transmission line is such thatthe axial velocity of microwave signals Conducted along the helical pathis approximately the same as or slightly slower than the velocityof theelectrons of the beam whereby the electrical field of the microwavesignals interact with the electron beam for amplification of themicrowave signals.

In the passage of the electron beam from an electron gun along thelength of the transmission line, there is a tendency for the electronsof the beam to deviate from n desired electron beam path and as a resultcauses an increased electron beam cross-section which eventually willbecome greater than the diameter of the helical transmission line. Thisdeviation of the beam is caused by the natural spaced-charge repulsionof electrons along the path of the helix, as well as by some types ofelectron guns which naturally projects a convergent electron beam. Inbecoming larger than the diameter of the helical transmission line, theelectron beam will transfer electron beam current to the helical linewhich is a feature undesired for maximum amplification as well asrequiring power dissipation on the helical structure. The use ofmagnetic fields to maintain electron beams uniform in diameter issubstantially well understood, which is to say that the requiredmagnetic field for any given application can be predicted. Heretoforethe solution to this problem has been solved by employing electromagnetscomprising a number of solenoid coils, ranging from one to four separatecoils, coaxial of the electron beam path and extending substantially theentire length of the traveling wave propagating structure. Thesesolenoids with their associated current supplying devices are arrangedto produce a substantially uniform magnetic field whose lines of liuxextend axially of the helical conductor for substantially its entirelength to insure a pencil-like beam throughout the entire length of thetransmission line.

In a number of traveling wave device applications these electromagnets,with the number of coils employed and the circuitry required toestablish the regulated current to ow therethrough to produce a desiredmagnetic field, become cumbersome. Further, even with all the currentregulating circuitry, the magnetic field produced by the electromagnetsoften varies enough to produce an undesired effect of beam expansion andthus yielding undesired power operating characteristics. In theseapplications where the known electromagnet type electron-optical systemsbecome cumbersome, it may become desirable to employ a permanent magnetto produce the re- ICC quired magnetic field distribution. From theviewpoint of economy of the magnetizing force, consideration must begiven to the D.C. power when a solenoid is employed and the weight ofthe magnet when a permanent magnet is employed, as well as the qualityof the result. It is required for use in electron-optical system thatthe magnetic field be uniform over the axial extent of the electron beamin the helical propagating structure. In many of these applications ithas been found preferable and more economical to employ permanent magnettype of electron-optical systems.

It is an object of this invention, therefore, to provide an improvedelectron-optical system of the permanently magnetized type to provide asubstantially uniform axial magnetic field; and another object is toprovide such an electron-optical system in combination with a travelingwave type of electron discharge device.

Still another object of this invention is the provision of a hollowcylindrical magnetic system capable of providing a substantially axialmagnetic field throughout substantially the entire length thereof.

A feature of this invention is the tapering of the interior wall of ahollow cylindrical permanent magnet in a manner to obtain a uniformaxial magnetic field over a substantial portion of the axial extent ofthe permanent magnet.

Another feature of this invention is the employment of a shell ofsaturable magnetic material coaxially of and within a hollow cylindricalpermanent magnet to assist in aligning the axis of the magnetic fieldwith the mechanical axis of the permanent magnet, and where such magnetis used in conjunction with a traveling wave electron discharge deviceto assist in making the axis of the magnetic field coincide with theelectron path of such device.

Additional features of this invention include the combination of a shellof saturable magnetic material and a cylindrical shell permanent magnetof uniform crosssection; a shell of saturable magnetic material and acylindrical shell permanent magnet having a tapered interior wall; and ashell of saturable magnetic material and a cylindrical shell permanentmagnet having a tapered exterior wall; each combination producing asubstantially uniform magnetic field in substantial coincidence with themechanical axis of the permanent magnet.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

Fig. 1 is a longitudinal section illustrating an application of anembodiment of this invention;

Fig. 2 is a curve illustrating the ideal magnetic field distribution forapplication with a traveling wave tube which the embodiments of thisinvention attempt to approach;

Figs. 3 and 4 are longitudinal sections illustrating other embodimentsof this invention; and

Figs. 5 and 6 are graphical representations of the axial magnetic fielddistribution for the various embodiments of this invention with andwithout the shell of the saturable magnetic material.

Referring to Fig. l, a traveling wave type of electron discharge devicel is shown to comprise a cylindrical housing 2 containing at one endthereof an electron gun unit 3, at the opposite end thereof an electroncollector 3a and intermediate thereof a radio frequency interactionsection 4. The housing 2 has mounted thereon and concentric thereto anembodiment of an electron-optical system 5 in accordance with theprinciples of this invention.

For application with a traveling wave type it is desirable to have amagnetic field distribution as illustrated in Fig. 2 or as close as maybe practically attainable. The

embodiments vof theelectron-optical systemSt shown .herein produce apractical magnetic field distribution which closely approximates thatshown in Fig. 2. The form shown in Fig: `l comprisesa:cylindricalfsheilof Vsaturahle magnetic material 1 74 and a cylindrical.permanent '.magnet 6, the latter havinga taperedinnersurfaces"forfproduo: tion of an axial magnetic fieldtomaintainfazsubstantially uniform electron beamv throughout thelength1ofV section 4.

Housing 2 maybe composedofceramicor non-magnetic metallic material asdesired.V Theiinternal structure ofthe' electron gun unit 3' mayfollowiany ofltheconventional structures presently. known in the' art.device 1 employed with the electron-optical system 5' shouldfollow'generally the. teachings of theV copending application-of J. H.BryantandT. J; Marchese, Serial No.

221,862, filed April 19, l95l,.now Patent No. 2,788,465.l Disclosedtherein is the structural arrangement of atraveling wave tube withthe'terminals for-the electrodes of the gun unit at yone end of the tube andthe terminals forV the radio frequency input and output connectionsatthe other end ofLth/e-tube. The input and output radio frequencyconnectionsldisclosed therein are of the coaxial type, but may beadapted to function with waveguidecoupling lines as disclosed in thecopending application of J. H. Bryant, A. G; Peifer, and R. W. Wilmarth,SerialV No. 305,538,`I filed August-21, 1952. It is desirable for thestructure of 'thetravelingwave tube employed with the electron-opticalsystem of this invention to have radio frequency input and outputconnections carried longitudinally'by the tube so that theelectron-optical system is not physically distributed by carrying suchconnections at right angles tothe devices.

Referring more particularly to interaction section 4, aV radio frequencytransmission line 8 preferably in lthe form of a helix 9 maybe supportedin a ceramic tubing positioned axially by the magnetic barrier members10 and 11 and the matching transformer sections 12 and 13located-respectively at the input and output ends of section 4. Itispreferred, however, to support helix 9 in a plurality of dielectricsupport rods 14 substantially as' disclosed in the copendingapplicationof R. W.V Wilmarthand B; D. McNary, Serial'No.-3'05,2f28,.filed August 19, 1952. Disclosed therein-are meansand-methods for manufacturing a rigidl transmission line or propagatingstructure while maintaining-,close mechanicalV tolerances withV every'turn of'the helicalA conductor being bonded to the dielectric supportrods. Preferably, transmission line 8 shouldbe coated forsubstantially-the firsthalf or a selectedportion thereof of its lengthtwith aquadag or other lossyrmaterial to minimize fiow of`reverseradiofrequency'waves along the conductor 9 and the electric field thereof.Thepropagatingstructureis not necessarily limited to the helicalconductor type but may assumev other configurations such as a pluralityof annular discs or plates, whereby the axial velocity of theradi'ofrequency energy is'preferably slig tly slower than the velocity of thebeam of the electrons projected from the unit 3.

The major advantageinan electron-optical system employing a hollowcylindrical permanent magnet is the economical value wherethe' weight ofsuch a magnet is not very great, the'D.-C, supply for a solenoid iselimi-. nated, anda magnetic field distribution is obtained which,approaches more closely the ideal magnetic distribution illustrated inFig. 2. However, it must be remembered that the electron beams used intraveling Wave tubes assume two different types'l of geometry. Themagnetic field requirements for each geometry differ only in the fielddistribution requirements at the ends ofthe magnet'. The first,.whichshallbereferred t-o astype l, relates to tubes withrectilinear electronfiow from cathode to collec.-

uaitcrramagnticfeld over a giveninterval withoutre;

- field region.

:gardofwhat occurswoutside that interval. This-is-.truev since thecathode may Ibe operated Within the uniform The second, referred to astype 2, relates to tubes using electron guns with converging electronbeams. For this application it is essential that the magnetic field beeliminated from the cathode-anode region. Also, for maximumefiicientutilization-of a magnetic field, a .type

" of focus'i'ng'that has been referredto generally as Brillouin focusingshould be used. For Brillouin focusing and arepreferably employedfrom aneconomy-standpoint for type l applications and also for some of thetype'2" applicaionswhere-the cathode-anode regionis specificallyconstructed to provide a shielding from magnetic fieldsandfarranged suchthat the pointwhere the electron beamAceasesconverging-andbecomes-parallel tothe axis occurs just/outsidethe-shielded'cathode-anode region. A longitudin'al section of this typeoffmagnet is'illustrated in Fig: 3Wherein shell 15 ismagnetizedlongitudinally resulting-inan axial magnetic field distribution.`axial'magnetic field-distribution illustrated byA curve 16 of Fig. 5indicates a drop-in magnetic field strength at the central'portionfda-of the magnet which isobjectionable intheapplication to atraveling'wavetube. Thereversals' 1Gb and i6c ofthe magnetic field atthe ends of the` cylindrical shell magnet are peculiar to permanent mag-Y netsand are shown byflux lines inV Fig. 3. As can be seen' thedirection-of thelmagneticV field actually reverses at'o'r nearV the endof themagnet. This reversal of. field is not objectionable for type lapplications but istobje'ctionable for type 2 applications unlessarranged Yin' a manner such that the reversalv occurs in the shieldedVp0rtionofthe cathode-anode region.

It was discovered that tapering the wall of the magnet from-the centeroutward as illustrated in Fig. 1, results in an' axial magnetic fieldsubstantiallyas shown in Fig. 6, curve'19," substantially eliminatingthedrop in magnetic fiel'dstrength' over the central portion of theApermanent magnet ofFig; 3, see curve 16offE`ig..5'. As can be seen,

theffieldisquite uniform' exceptforthe slight rise in the centerv ofthe'magnet. This' raise can be eliminated byl judicial grinding'away ofthe' sharp discontinuity at point 17` of magnet 6 Whereinthe innermagnet wall is tapered. from the' centerV outward.l

In-Fig. 4 the' outer diameterjof the magnet 18"i's shown.

tapered from"thecenterto'the' ends.V It was found that thema'gneti'c4field distribution 'for magnet I8`i`s similar tov the uniform diameterVcylindrical'. permanentV magnet as' illustrate'dby curve 16 of FigfS'.,

Experiments with sample magnStS'ofthe type shown in Fig; l demonstratedthat' the alignment ofthe magnetic.

fieldf'axis with the' mechanical axis ofthe magnet did not.

always-agree; Afurtherdifiiculty encounted'was-that the direction .ofthe magnetic fieldat'oneend of thefmagnet shell 7isaturatesmagnetiallyat. magnetic field strengths` equaltoor lessthan the-minimum field whichone. wishesto. control.. 'The4 employment-of such.V a thin; shell ofsaturableV magnetic material-not only; aids inzthe :alignment butyasrcndera-the field strengthv more uniform: in; all; i.

The resulting K embodiments of cylindrical shell magnets illustrated inFigs. 1, 3, and 4. Reference to Figs. 5 and 6 and curves 20 and 21,respectively, illustrate the behavior of the axial magnetic field whenthe shell 17 is employed in conjunction with the embodiments of thecylindrical shell magnet shown in Fig. 1 and Figs. 3 and 4, the curves16 and 20 being typical for both forms of magnets shown in Figs. 3 and4. Such a magnetic field distribution as illustrated herein for each ofthe separate embodiments illustrates that the various embodiments may beinterchanged to function as a portion of the electron-optical system asshown in Fig. 1 with equivalent practical advantages. However, it can bereadily seen by those skilled in the art that it would be preferable toemploy the embodiment shown in Fig. l for an application wherein themagnetic field distribution is required to rigidly maintain the electronbeam uniform throughout the length of interaction section 4.

While I have described above the principles of my invention inconnection with specic apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims.

claim:

1. In a device of the traveling wave tube type which includes means forproducing an electron beam for flow along a given path and a radiofrequency propagating structure disposed adjacent said path to enableinteraction between the electrons of said beam and radio frequencyenergy propagated along said structure; an electron-optical systemdisposed concentrically of said path comprising a hollow cylindricalpermanent magnet magnetized longitudinally in a manner to produceoppositely polarized poles substantially at the ends of the cylindricalform of said magnet and a thin cylinder of saturable magnetic materialdisposed concentrically within said magnet and about said path to orientthe axis of the developed magnetic field with the path of said beam,said permanent magnet comprising a cylindrical shell having across-section tapering from the center toward its ends.

2. In a device according to claim 1, wherein the inner diameter of said'cylindrical shell is constant and the outer diameter thereof varies ina manner along its length such that the outer surface is tapered fromthe center to the ends.

3. In a device according to claim 1, wherein the inner diameter of saidcylindrical shell varies in a manner along its length such that theinner surface is tapered from the center to the ends.

4. In a device according to claim 3, wherein the central portion of saidinner surface is slightly rounded to reduce the abrupt change of thecontour of said inner surfare.

5. In a device of the traveling wave tube type which includes means forproducing an electron beam for ow along a given path and a radiofrequency propagating structure disposed adjacent said path to enableinteraction between the electrons of said beam and radio frequencyenergy propagated along said structure; an electron-optical systemdisposed concentrically of said path comprising a hollow cylindricalpermanent magnet magnetized longitudinally in a manner to produceoppositely polarized poles substantially at the ends of the cylindricalform of said magnet and a thin cylinder of saturable magnetic materialdisposed concentrically within said magnet and about said path to orientthe axis of the developed magnetic field with the path of said beam,said permanent magnet being tapered in cross-section toward at least oneend thereof.

6. In a `device of the traveling wave tube type which includes means forproducing an electron beam for flow along a given path and a radiofrequency propagating structure disposed adjacent said path to enableinteraction between the electrons of said beam and radio frequencyenergy propagated along said structure; an electron-optical systemdisposed concentrically of said path comprising a hollow cylindricalpermanent magnet magnetized to produce oppositely polarized polessubstantially at the ends of said magnet having the cross-section of aportion thereof tapering toward at least one of its ends.

7. In a device according to claim 6, wherein the inner diameter of saidmagnet varies in a manner along its length such that the inner surfaceis tapered from the center portion thereof toward its ends.

8. A permanent magnet comprising a hollow cylinder having a crosssection tapering from the center towards its ends magnetizedlongitudinally to produce oppositely polarized poles substantially atthe ends of said cylinder and a saturable magnetic shell of uniformcross section disposed to be concentrically within said cylinder,substantially coextensive of said cylinder and in contact with at leastthe central portion of said cylinder for cooperation therewith toproduce a substantially uniformly dis tributed magnetic field axially ofsaid cylinder.

9. A magnet according to claim 8, wherein the inner diameter of saidcylinder varies in a manner along its length such that the inner surfaceis tapered from the center portion toward its ends.

References Cited in the le of this patent UNITED STATES PATENTS2,259,531 Miller et al. Oct. 21, 1941 2,267,545 Wente Dec. 23, 19412,300,052 Lidenblad Oct. 27, 1942 2,431,077 Poch Nov. 18, 1947 2,433,682Bradley Dec. 30, 1947 2,442,975 Grundmann .lune 8, 1948 2,602,148 PierceJuly 1, 1952

